1. Field of the Invention
This invention relates to the ultrasound treatment of body tissue. It is disclosed in the context of prostate ablation. However, it is believed to be useful in other contexts as well.
2. Description of the Related Art
The ablation of tissue using ultrasound is known. In order to achieve ablation safely and optimally, knowledge of the power which must be supplied to the therapy transducer is essential. Generally there are guidelines for the required power, but these guidelines are not absolutely accurate, particularly where the tissue between the transducer and the treatment site is not homogeneous or where the treatment site is deep seated, that is, remote from the transducer. In the case of prostate ablation by focused ultrasound, for example, calcium or a fatty layer or both can lie between the colon, where the treatment transducer is ordinarily introduced into the body and resides in such treatment, and the treatment site in the prostate.
If the distance from the transducer to the treatment site is defined as L, then the intensity of the ultrasound field at the treatment site, I.sub.site, can be defined as EQU I.sub.site =I.sub.o .epsilon..sup.-.mu.L
where I.sub.o is the free field intensity of the ultrasound, .epsilon. is the base of the natural logarithms, and .mu. is the so-called attenuation coefficient of the intervening tissue or other material. For ultrasound at a frequency of 4 MHz and water, .mu.=0. That is, ultrasound at this frequency passes essentially unattenuated through water. Thus, for any water path length, I.sub.site =I.sub.o. For other tissues typically encountered between the wall of the colon and the urethra where it passes through the prostate, .mu. has been determined empirically to lie somewhere in the range of 0.64/cm. However, in vitro studies of human tissues lying in this region have established that the attenuation coefficient can vary widely. Figures for .mu. as high as 1.0/cm and as low as 0.3/cm have been encountered. With this broad range of values, it will be appreciated that a rather small variation in L can result in site intensities well below or well above those required to achieve prostate ablation, for example.
At ultrasound frequencies below about 15 MHz, the attenuation coefficient of warm-blooded animal tissues can be considered to be proportional to frequency. This is the so-called low frequency range generally considered the diagnostic (for example, visualization) and therapeutic range for ultrasound tissue ablation. It has been established that the attenuation coefficient in this range can be measured using thermocouples. There is, for example, Fry, William J. and Ruth Baumann Fry, "Determination of Absolute Sound Levels and Acoustic Absorption Coefficients by Thermocouple Probes-Experiment", 26 J. Acous. Soc. Amer., 311, May, 1954. These observations are combined in the present invention to provide an apparatus and a method for determining the treatment transducer's output in the free field in order to provide sufficient ultrasound intensity at a site to achieve a desired therapeutic effect.